When I had the 2 Dell PowerEdge dual Xeon servers hooked up to my APC 1500 UPS (pretty big and heavy - might be same one you have), the lights would flash, meaning it was overloaded, but no beeps.
Now that I have just the Beerinator on there, the lights don't even flash...so I'm getting bazillions more credits with less juice!

Try a manual reset of the UPS; I had to do this once after a "brown-out" power outage here. (Yep, SoCal is pulling so much juice you could probably almost light the rest of the nation with our grid if you took out NY and Chicago!)
You power the unit down, then unplug it from the wall. Then, unplug the little "plugin" in the back that is like a "key" to the battery. This disconnects the internal battery from the unit. Hold the power button "on" with your finger for like 20 to 30 seconds. Then hook it all back up and turn it on...leave it on for about 1 min to go through its self-check cycle. Then turn your HAMMER on again. See if that takes care of the beeping.